Field of first flux entry and pinning strength of superconductors for RF application measured with muon spin rotation

The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry $H_{entry}$ and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation ($\mu$SR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400{\deg}C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure $H_{entry}$ of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.